The invention relates to a method for protecting force transmitting machine elements forming pairs, which are subject to oscillating loads, against fretting corrosion. Especially machine elements having the same composition or characteristics are subject to such fretting corrosion.
In force-transmitting machine elements forming pairs, for instance in power plants or drive trains made of titanium, especially in the area of the blade bases or blade feet, fatigue failures occasionally occur already at loads far under the fatigue strength limit. Such fatigue failures start at surface areas subject to fretting corrosion damages. The cause of such damages is a complex surface loading or stress which is called "fretting fatigue stress or load".
This fretting fatigue stress is composed of a surface pressure or compression and an alternating shear stress or strain due to smallest chafing motions, also referred to as "slip" between the paired or pairing surfaces. The arising shear strains are unexpectedly high. The fretting fatigue load or stress can damage the machine element in two ways. On the one hand, fretting corrosion can arise as surface damage. On the other hand, a reduction of the fatigue strength limit results in machine elements subject to oscillating or dynamic loads if two metals rub against each other. The movement is cyclically oscillating with a movement direction in parallel to the junction surface forming said pairing surface. The oscillating amplitude is so small that surface damage by abrasion results, but no abraded matter or particles can be removed from the contact or pairing surfaces.
The fretting corrosion damage allows making inferences or conclusions regarding the fretting fatigue load or stress. If the fretting fatigue stress exceeds the strength limit of the material, microcracks result in the surface. As a result, tribologically, chemically activated particles may be emitted from the surface, which particles spontaneously react with the oxygen of the atmospheric air or with oxygen used as a working medium. The thus arising oxidic products of abrasion led to the term "fretting corrosion".
The following observations may be made about the extent of the surface destruction:
the boundary surface forming one of the pairing surfaces is subject to erosion or wear and tear which increases with the number of fretting or frictional oscillations,
the boundary surface erosion or wear and tear increases with increasing surface pressure,
the initial condition of the boundary surface practically does not influence the extent of the destruction, and
lubricants have only a slight effect or influence on the reduction of fretting corrosion damages.
If a machine element subject to oscillating or dynamic loads is additionally subjected to a fretting fatigue load on the surface, then a reduction of the fatigue durability or strength results. In practice, the term "fretting fatigue fracture" is used for the fracture of an oscillatingly loaded structural element. Such fracture originates from said fretting fatigue loaded surface. The fretting fatigue fracture is reliably distinguished from other fatigue fractures by oxidic abrasion products and by a so-called "nose" at the starting point of the fracture.
The largest oscillating load or dynamic stress applied to a structural machine element which does not cause any fretting fatigue fracture is termed "fretting fatigue durability or strength".
The danger of fretting fatigue fractures in oscillatingly or dynamically loaded machine elements or structural groups of such elements always exists if these elements are so arranged that oscillating relative displacements (or "slip") occur between the paired surfaces under load conditions. Thus, fretting fatigue fractures occur quite frequently, among other situations in sealing inserts for rotors of turbo-compressors, and especially in power plants equipped with titanium blades where these fractures occur in the area of the blade bases.
For avoiding such fractures, various steps have already been recommended (Forschungshefte-Forschungskuratorium Maschinenbau e.V. Heft 56, 1976, pages 131 to 139 "Reibkorrosion-Abschlussbericht"). Such steps include, for instance, avoiding force transmitting paired junction surfaces, e.g. by welding, soldering or by adhesive connections rather than contraction or clamping connections; reduction of the fretting fatigue load or stress by various measures, e.g. by making the strain or tension differences of the paired elements as small as possible, or by separating the fretting fatigue load from the structural component load by means of relief notches on the connecting elements, whicn connect the junction parts. For example, axial and tangential relief notches may be provided in fitted bolts for disk clutches.
It is also known from German Patent Application No. DE-AS 2,836,334 to provide the junction surfaces of the machine parts with connection elements having a different crystal structure and a larger hardness than the parts to be joined, in order to increase the fretting fatigue durability or rather decreasing the fretting corrosion of the junction parts. These connection elements are however, discrete structural components and are comparatively massive and hence their usability is only limited. Such connection elements are unsuitable for power plants with titanium blades for avoiding or reducing the fretting corrosion, especially at the blade bases, due to the reduced fretting fatigue durability of the material under oscillating fretting loads or dynamic fretting stress.